The position of a device or another target can be determined by several different methods. Satellite-based systems are known, which use several satellites orbiting the earth and transmitting signals. The signals transmitted by the satellites are received, and on the basis of the signal reception, various measurements are taken to determine the position of the target. The signal measurement is taken either in the device whose position is to be found out, and/or on an earth station with a known position, such as a base station of a mobile communication network. Such satellite positioning systems include, for example, the GPS (Global Positioning System) and the GLONASS (Global Orbiting Navigation Satellite System). Satellite-based positioning has, for example, the advantage that under favorable conditions, the position can be determined at a relatively high precision. Even relatively inexpensive receivers can be used to determine the position at a precision of even few meters. This is often the case in exposed places, where there are hardly any obstacles to attenuate the satellite signals. Under poor conditions, however, when the signal strength is weak, positioning on the basis of the satellite signals is not necessarily even possible. Such a situation often occurs in cities and particularly indoors, where the signal has to pass through the structures of a building. Furthermore, if the receiver cannot receive signals from a sufficient number of satellites, the position can thus not be determined by means of the satellite positioning system only.
At present, the position of a mobile communication device can be determined by base stations. Thus, measurements are taken on the basis of signals transmitted between the base stations of the mobile communication network and the mobile communication device. In the positioning, for example differences in signal propagation times between the mobile communication device and the different base stations are utilized. In such a system, the positioning accuracy is typically in the order of 100 meters or poorer, depending on the size of the range of the base station, i.e. the size of the cell. The smaller the cell size, the better the positioning accuracy. The size of a cell is not necessarily the same but it may vary. Normally, there are more base stations, i.e. the cell size is smaller, in densely populated areas, such as cities, whereas in regions with less population there are fewer base stations, i.e. the cell size is larger. If directional antennas are used at the base stations, the positioning accuracy can be improved to some extent. An advantage in such base-station based positioning is, for example, the fact that its application does not require the use of a separate positioning receiver, but a mobile communication device can be used for the positioning.
Positioning devices have also been developed, in which the above-presented methods have been combined. In such a hybrid system, both satellites and base stations are used for determining the position of the device.
In satellite-based positioning, the signals from at least three different satellites must be received to produce a three-dimensional position solution (x, y, z: latitude, longitude and altitude), if the exact time data is known in the receiver. For example in satellites of the GPS system, highly accurate atomic clocks are used, whose working accuracy is controlled at earth stations. If necessary, the time data of the satellite clocks can be adjusted on the basis of measurements taken by these earth stations. In conventional commercial positioning receivers, however, it is not possible to use such atomic clocks, e.g. for the reason that they are extremely valuable and require relatively constant ambient conditions to secure their reliable operation. In this case, the signal of a fourth satellite is still needed to determine the time error as well.
In base-station based positioning, the mobile communication device must receive the signals of at least two base stations to be able to determine the position of the receiver in two dimensions (x,y: latitude and longitude). On the other hand, base-station based positioning can also be performed vice versa; that is, the signal of the mobile communication device is received at two or more base stations and the two-dimensional positioning is performed on the basis of the signals received by the base stations. Three-dimensional positioning is not possible in base-station based positioning systems known at the present.
In positioning systems of prior art, the position is to be determined iteratively by means of non-linear equation sets. Furthermore, it is possible to use overdetermination; that is, there are more equations than unknown variables to be solved. In practice, this means the number of received signals exceeds the minimum. For solving the equation sets, it is possible to use, for example, the least mean squares method and to look for a minimum. The problem is, for example, that if a default position has been used and it is significantly incorrect, or if different measurements are weighted in an inadvantageous way, the iterative computation may result in the finding of an incorrect minimum point. This means that the determined position is not correct.